Zone selective protection, also called zone selective interlock (ZSI) is a method of reducing the impact on the various power distribution devices in a power distribution network in the event of a short circuit fault by shortening the time taken to eliminate the fault as much as possible. To be specific, supposing a fault occurs in a multi-stage power distribution network supplied by a single power source, then every circuit breaker which detects the fault must report detection of the fault to the circuit breaker lying upstream of itself, i.e. send an interlock signal. Thus, only those circuit breakers which receive no report are determined as being the circuit breakers closest to the fault point and perform instantaneous protection (e.g. delay time t=50 ms), i.e. perform a disconnecting action with the shortest delay time. The circuit breakers which receive a report perform timed protection, also called “locking”. By “timed protection” or “locking” is meant: if the fault has still not been eliminated (e.g. the protecting action of a circuit breaker closer to the fault point fails) after a preset delay time (e.g. 100 ms), the circuit breaker opens, otherwise it maintains the connection. Clearly, ZSI enables selective actions of circuit breakers.
The increased complexity of power distribution networks with multiple power sources means that the simple ZSI described above is not suitable for use in such networks. In view of this, directional zone selective interlock (DZSI) has been proposed, the basic principle thereof being as follows: the fault point is identified on the basis of the relationship among the directions of currents flowing through the various circuit breakers when the fault occurs, and a delay time is then determined for the action of each circuit breaker in order to eliminate the identified fault. The advantage of this method is that selective protection can be achieved more reliably even if the network contains multiple power sources or an active load.
There are many methods for implementing DZSI in the prior art. A method for implementing DZSI is disclosed in Chinese patent application No. 200910266751.7, disclosed on Jul. 7, 2010. The method in question used one independent processor to manage the various circuit breakers in a power distribution network, i.e. the processor monitored the fault current direction of each circuit breaker via a communication path connected to each circuit breaker, and on this basis indicated a delay time for the action of the relevant circuit breaker.